Fibrous materials, especially nanofibers, are a widely used industrial material, which in recent years have been the subject of numerous research and development efforts. A fiber can be defined as a three-dimensional structure that is much larger in one dimension then the other two. If an arbitrary object can be described by three characteristic length scales, a, b and c, then a fiber is described by a>>b and c. Frequently, a fiber takes the form of a long, cylindrically symmetric strand that has a circular cross-section that does not change significantly along the z-axis. Fibers are manufactured from all major materials classes (polymers, metals, ceramics and glasses) and are utilized in virtually every major industry in some form or another. They can be used in the form of single fibers, felts, textiles, cables, or reinforcing elements in composites, etc.
Fibrous materials provide enhanced flexibility, improved material properties, ultra-high surface area, chemical reactivity, increased strength, and the ability to transfer tensile loads using less material/volume. Despite these benefits, individual fibers are especially susceptible to damage, so they may often be used in combination with other materials. Multiple fibers may be combined to form textiles, metal wires, cables, polymeric cables, ropes, etc. Multiple fibers may also be used in fiber reinforced composites, wherein multiple fibers/textile layers are embedded in another material. This allows assembly of a large object with the properties of a fiber, wherein the surrounding material may provide protection, isolation, etc. Fiber reinforced materials may include fiberglass, carbon reinforced composites, metal and ceramic matrix composites etc.
Current research in the area of fibrous materials is focused on development of fabrication techniques that can produce nanofibers. Currently, nanofibers are produced by chemical synthesis methods, glass-drawing techniques, and modified extrusion strategies. Chemical synthesis methods rely on the controlled growth of a fibrous structure, in a liquid or gas environment, to produce a fiber in an atom-by-atom fashion. These methods are frequently used to produce single crystal whiskers of various inorganic materials, the best known of which is carbon nanotube synthesis. These techniques produce materials with extraordinary properties, but have proven entirely incapable of fabricating a continuous monofilament structure. In fact, short carbon nanotubes grown by these methods are currently being used as a feedstock for the production of continuous fibers via other extrusion-based methods.
Glassdrawing techniques have been utilized to produce continuous nanofibers from a limited selection of inorganic glass and polymer materials. Furthermore, a highly modified extrusion method known as electrospinning is often utilized for fabricating long nanofibers. Electrospinning involves the extrusion of a highly viscous polymer precursor through a relatively large aperture. This occurs simultaneously with the application of high voltage between the aperture and an appropriately positioned collector plate. Electric field effects cause the fluid to form a “Taylor cone” which is essentially a fluid instability at the center of the meniscus. A microscopic jet of precursor material emanates from the Taylor cone and is pulled to the collector plate by the applied electric field. The fluid jet propagates in a pseudo-random fashion, producing stretching and bending motions that further reduce the diameter of the fiber, which is eventually deposited on the collector plate. This process often produces a “nano-felt” material. Efforts to produce truly spoolable, continuous nanofiber with electrospinning focus on modified collection strategies such as spinning mandrels or electrode arrays. This allows the collection of highly aligned nanofibers, but does not produce a nanofiber that can be manipulated as a single continuous strand.
The difficulty of nanofiber production lies primarily in geometrical considerations, and the particularly small scale of the cross sectional features. Accordingly, the present invention is directed to a fiber fabrication method that can reliably produce the necessary nanoscale dimensions, yet allow the fiber length to be extended to arbitrary lengths. As demands increase for continuous nanofibers, the need arises for improved methods of producing these fibers, especially nanofibers having submicron dimensions.